In recent years, the volume of communications traffic via radio has increased. However, the frequency band used in radio communications is limited. Accordingly, it is desired that the utilization ratio of the frequency band should be improved. As a method for improving the utilization ratio of the frequency band, it is possible for example to adopt a configuration in which every base station can allocate the entire band of the frequency used in radio communications to a mobile station, as in Orthogonal Frequency-Division Multiplexing (OFDM) of downstream links or the single-carrier transmission of upstream links in Long Term Evolution (LTE). However, in communications in which each base station allocates the entire band of the frequencies used in radio communications, a mobile station located in one cell may interfere with the communications of a mobile station located in a nearby cell. Moreover, the communication state of a mobile station located at an edge of a cell may deteriorate not only because the received power at the communicating base station becomes weak in the upstream link due to being a long distance from the base station but also because the interference caused due to a mobile station belonging to a nearby cell becomes large. Accordingly, the average throughput or the coverage in the whole system may deteriorate due to interference with a cell caused by a mobile station located at an edge of a nearby cell. Further, when the service area of a cell is divided into several sectors, the above problem may occur due to the interference between sectors. For example, when the service area of one cell is divided into three sectors, as illustrated in FIG. 1A, in addition to interference among the cells a-d, interference among the sectors within the cells a-d may also occur.
In view of the above problems, Inter-Cell Interference Coordination (ICIC) is known as a method for preventing interference between nearby cells or sectors. In ICIC, the frequency band is divided into several bands, as illustrated in FIG. 1B. In the example of FIG. 1B, the frequency band is divided into three bands. The base station 1 (1a-1d) selects for each sector a band to be allocated to a mobile station located at an edge of a sector. At this time, the base station 1 selects bands in such a manner that the band allocated to a mobile station located at an edge of a sector will not be the same band as that of a nearby sector. For example, the base station la of the cell a allocates the band 1, the band 2, and the band 3 to mobile stations located at the edges of the sector 101, the sector 102, and the sector 103, respectively. Moreover, the base station la of the cell a selects bands in such a manner that the band allocated to a mobile station located at the edge of the sector will not be the same band as the band of a nearby sector that is also between the sectors of the cells b-d and the sectors of the cell a. Here, FIG. 1B illustrates an example in which the base station 1 allocates bands to mobile stations located at the edges of the sectors, where “sector x01” indicates that the last two digits of the sector ID are “01” and the third or larger digits are arbitrary. The same applies to “sector x02” and “sector x03”. In other words, the band 1 is allocated to the mobile stations located at the edges of the group of sectors 101, 201, 301, and 401. The band 2 is allocated to the mobile stations located at the edges of the group of sectors 102, 202, 302, and 402. Then the band 3 is allocated to the mobile stations located at the edges of the group of sectors 103, 203, 303, and 403, respectively. Here, whether a mobile station is located at an edge of a sector is determined, for example, by a ratio of the path-loss (PL1) between the base station that forms the sector in which the path-loss is the smallest and the mobile station to the path-loss (PL2) between the base station that forms the sector in which the path-loss is the second smallest and the mobile station. In other words, the base station determines that a mobile station whose PL1/PL2 ratio value is larger than a threshold value is located at an edge of a sector. Here, a path-loss is a value indicating the magnitude of the propagation loss. Normally, the sector in which the path-loss is the smallest is the sector in which the mobile station is located. A small PL1 value indicates that the communication state with a base station forming a communicating sector is good. A small PL2 value indicates that the interference of a mobile station with other mobile stations located in a nearby sector is large.
It is a known technology to allocate to a mobile station the frequency allocated to a nearby base station when the mobile station is located near the center of a cell, and to allocate a frequency different from the frequency allocated to a nearby base station when the mobile station is located near the boundary of the cell of the nearby base station. Alternatively, it is also a known technology to schedule an uplink resource with reference to the uplink control information generated on the basis of the information of the interference amount among cells. It is a known process to determine the transmission power for each frequency band used for the data transmission on the basis of the power characteristic of the radio channel measured from a pilot signal. Furthermore, it is a known scheduling process to form a group of mobile stations on the basis of the positional information of those mobile stations, and to allocate a communication slot of the same time channel or the same frequency channel to a mobile station that belongs to the same group.